Referring to FIG. 1, an example of a conventional phase locked loop circuit 10 is shown. The circuit 10 generally comprises phase frequency detector 12, a charge pump 14, a voltage controlled oscillator (VCO) 16 and a divider 18. The circuit 10 is used to multiply a reference signal REFCLK having a fixed frequency, received at an input 24, by some multiple set by the divider 18. The frequency detector 12 is coupled to the VCO 16 through the charge pump/filter 14. The divider circuit 18 has an input 28 that receives a feedback of the signal CLK presented at an output 30 of the VCO 16. The divider 18 presents a signal to an input 32 of the frequency detector 12. Errors coupled through the charge pump 14 cause the VCO 16 to change the frequency of the signal CLK to minimize the error. VCO frequency errors may be managed by the circuit 10. The nominal frequency of operation of the signal CLK will be the frequency of the reference signal REFCLK multiplied by the divider ratio.
The VCO 16 is an integral part of the PLL 10. The gain of the VCO 16 (i.e., the frequency of the signal CLK vs. the change in the input control voltage Vcontrol), and the frequency range of the VCO 16 are two very important design parameters. The gain of the VCO 16 is important to determine the bandwidth of the PLL 10. Also, the gain of the VCO 16 has a significant impact on jitter (i.e., high frequency semi-random displacement of a signal from its ideal location). The frequency of the VCO 16 determines the operating frequency of the PLL 10.
FIG. 2 illustrates a diagram of a conventional VCO 16. The VCO 16 is usually made up of a voltage to current converter 40 and a current controlled oscillator (ICO) 50. The converter 40 comprises a current mirror 42, a comparator 44, a transistor 46 and a resistor R. The frequency of operation of the VCO 16 is directly dependent on the current Ivco received by the ICO 50. The current Ivco is the voltage of the signal Vcontrol divided by the resistance R. The higher the current IVCO, the higher frequency of the signal CLK. The current of the ICO 50 depends on the value of resistor R. Also, the gain of the VCO 16 depends on the value of the resistor R.
The value of the resistance R determines the gain of the VCO 16 (i.e., slope of the F-V curve). Changing the gain of the VCO 16 will also change the frequency of operation of VCO 16. This may put the frequency of the VCO 16 out of the required range.